Method of determining the biological active urotensin II in a sample of a body fluid, comprising contacting the sample with polyclonal and/or monoclonal antibodies which have been elicted by immunisation with a cyclic peptide having the sequence: having a disulfide bridge formed between Cys 5 and Cys 10, and comprising the artificial sequence βAla-Cys, optionally conjugated with a carrier peptide. The obtained antibodies are then either purified or characterised by their binding to cyclic human urotensin II and can be used in a binding assay for a diagnosis of cardiovascular diseases, heptarenal syndrome, cirrhosis, diseases involving haemodynamic alterations, pathological conditions involving vasocontrictive substances.